1. Field of the Invention
The present invention generally relates to a developing device used in an electrophotographic or electrostatic image forming apparatus and, more particularly, to the developing device of a type utilizing a developing material of two-component type including toner and carrier.
2. Description of the Prior Art
In the prior art developing device operable with the use of the toner material of two-component type including toner and carrier, the toner mixing ratio, i.e., the ratio of mixture of the toner relative to the carrier, which is also referred to as the toner density, is one of important factors in stabilizing developing characteristics to accomplish a reproduction of a high quality image. Accordingly, in order to produce a highly favorable image quality, it is necessary to accurately detect the toner density in the developing material and then to strictly control the amount of supply of toner according to a change in toner density thereby to maintain the toner density in the developing material at a constant value at all times.
As a means for accomplishing a toner density control (ATDC), a magnetic ATDC has generally been employed in which the toner supply control is carried out by detecting the magnetic permeability which varies with a change in relative density of magnetic carrier particles. However, if the fluidity of toner is enhanced as a means for accomplishing a favorable reproduction of a half-toned image with a reversal developing system, the bulk density tends vary as a result of a stirring of the developing material and, therefore, the magnetic ATDC cannot be employed.
Under these circumstances, an optical ATDC is generally employed wherein infrared rays of light of 890 nm in wavelength are radiated from an infrared light emitting diode towards the developing material and the rays of light reflected from the developing material are detected by a photodiode. Where this optical ATDC is employed, the infrared rays of light radiated undergo a total reflection with the toner of cyan, magenta or yellow color and the same is true with the toner of black color provided that the black toner material does not employ carbon, but pigments of cyan, magenta or yellow. However, the infrared rays of light tend to be absorbed by carrier particles. In view of this, the toner density can be detected by detecting the rays of light reflected from the developing material.
More specifically, a difference between an output obtained when a reference light is radiated to the photodiode and an output obtained when the reflected light while the developing material is of a normal density is used as a reference value which is subsequently compared with a difference in output obtained at the time of detection so that, when the difference obtained at the time of detection is lower than the reference value, the toner density can be determined to be low and the toner is therefore supplied.
However, in the prior art optical ATDC, it often occurs that the luminosity of the infrared light emitting diode which serves the light source tends to vary with time and/or that an output from a light receiving element tends to vary with change in temperature. Therefore, unless any correction is made to such variation, the toner density cannot be detected accurately.
In view of the foregoing, according to, for example, the Japanese Laid-open Patent Publication No. 63-177174 published in 1988, there is disclosed the use of a standard reflection density pattern for reflecting, in response to a radiated light, rays of light in an amount appropriate to a predetermined reference value for the density of the developing material, so that a reduction in luminosity of the light source with time can be compensated for.
It is also well known that, while an incandescent lamp is employed for the light source and a sensor windowpane is employed in the form of a dichroic mirror, and by selectively positioning in front of a light receiving element one of a filter capable of passing therethrough infrared rays of light and a filter capable of passing therethrough reference rays of light of a wavelength shorter than that of the infrared light, an output detected as a result of reflection from the developing material can be compensated for by a reference output from the sensor resulting from the reference light.
However, according to the prior art, the sensor requires the standard reflection density pattern or the filters to be incorporated therein together with an operating means therefor, rendering the system as a whole to be complicated and bulky accompanied by an increase in manufacturing cost.